The present invention relates generally to orthopedic appliances and, in a preferred embodiment thereof, more particularly provides a uniquely operative knee brace whose thigh and calf support structures are caused to laterally shift relative to each other, in response to extension and flexion of the leg, to controllably counteract the tibial shifting forces caused by the absense of or damage to the anterior cruciate ligament.
During extension of the normal human leg toward its fully straightened position the anterior shifting force imposed on the tibia caused by the quadriceps muscle is counteracted by the anterior cruciate ligament to prevent the tibia from being anteriorly shifted away from its normal position relative to the femur. Loss of or damage to the anterior cruciate ligament permits the quadriceps muscle to cause this undesirable tibial shift unless a counteractive force is imposed on the tibia by alternate means such as orthopedic appliance.
An example of a conventional attempt to artificially create this counteractive force is represented by the anterior cruciate ligament brace disclosed in U.S. Pat. No. 4,697,583 to Mason et al. In the Mason et al knee brace, a four-point static leg load system is used in an attempt to maintain a posteriorly directed resultant force on the tibia, while at the same time maintaining an anteriorly directed resultant force on the femur, the two resultant forces creating a counteractive shear force across the knee which is oppositely directed relative to the tibia-femur shear force across the knee created by the quadriceps muscle as the leg is extended toward its straightened position.
To achieve this four-point leg loading format, the Mason et al brace utilizes a pair of rigid thigh support members which extend along opposite sides of the thigh, and a pair of rigid calf support members which extend along opposite sides of the calf. The inner ends of the thigh support members are pivotally connected to the inner ends of the calf support members by a pair of polycentric hinges, positioned on opposite sides of the knee, whose pivotal motions are designed to closely mimic that of the human knee joint.
A generally U-shaped anterior thigh cuff member is secured to the outer ends of the thigh support members, and a generally U-shaped posterior calf cuff member is secured to the outer ends of the calf support members. A posterior thigh force strap is interconnected between the thigh support members and is positioned longitudinally between the anterior thigh cuff member and the hinges. In a similar fashion, an anterior tibial force strap is interconnected between the calf support members and is positioned longitudinally between the posterior calf cuff member and the hinges.
With the leg in its fully extended position, the Mason et al brace is installed by first aligning the hinges on opposite sides of the knee, with the thigh and calf support members respectively extending along opposite sides of the thigh and calf. The thigh and calf force straps are then respectively tightened against the back of the thigh and the front of the tibia.
The tightening of these two straps sets up two three-point force sets along the leg--the first force set comprising the posteriorly directed forces from the anterior cuff and strap members which oppose the anteriorly directed force from the posterior strap, and the second force set comprising the anteriorly directed forces from the posterior strap and cuff members which oppose the posteriorly directed force from the anterior strap. As a result, with the leg in its extended position, the two force straps cooperate to create a shear force across the knee joint which is directly opposite that created by the quadriceps muscle
This counter shear force created by the posterior thigh and anterior tibia straps in the Mason et al brace is static. The force is greatly limited and cannot appreciably change as the leg is flexed or extended. Thus the straps cannot compensate for the increased tibial displacement force caused by the quadriceps muscle as the leg is extended. The direction of force of the two straps, when the leg is flexed at 90.degree., tends to cause the entire brace structure to be undesirably shifted distally along the leg such that the brace must frequently be tugged upwardly to keep it in place. This, of course, decreases the wearing comfort of the brace and markedly decreases its effectiveness as a mechanical substitute for a missing or damaged anterior cruciate ligament. Attempts have been made to frictionally inhibit this distal shifting of the brace by securing it to a tubular undersleeve formed from a resilient material. However, in actual use it has been found that the brace still tends to distally migrate downwardly along the leg in response to flexure thereof.
Another problem associated with this type of statically loaded brace is that it does not accurately track (in a reverse sense) the quadriceps muscle-induced shear force across the knee joint. Specifically, the quadriceps muscle creates this shear force beginning at a leg flexion angle of approximately 40.degree. and significantly increases the force as the leg is moved closer and closer toward its fully extended position. With the leg between its fully flexed position and a flexion angle of approximately 40.degree., however, the quadriceps muscle exerts only a relatively slight anterior displacement force on the tibia. In contrast, the Mason et al brace creates an almost static shear force across the knee joint which actually increases slightly as the leg is flexed. This is contrary to the required force, which should increase as the leg extends. At best, therefore, this type of conventional brace only provides a minimal amount of the counter shear force of the anterior cruciate ligament which it is designed to replace or supplement.
In view of the foregoing it is an object of the present invention to provide a knee brace which eliminates or minimizes the above-mentioned and other limitations and disadvantages associated with conventional brace structures intended to function as an artificial anterior cruciate ligament.